Don't Abuse the Dead (Hadith No. 1175)


Bismillah.
Narrated 'Aisha :
The Prophet (p.b.u.h) said, "Don't abuse the dead, because they have reached the result of what they forwarded."
Abusing here refers to verbal abuse, using harsh words against a person.
Most scholars say that this is for the believers, i.e. don't abuse the dead believers. But anyhow, taking the hukm (commandment) of the Prophet (SAW), one must be careful as to not talk bad about any dead person, especially the believers.
This also implies not judging them, i.e. not deciding where they ended up. Because. We. Don't. Know.
No matter how pious you knew the person to be, you are not allowed to say (even about a martyr) that he/she definitely went to Jannah, or in the case of a wicked one, that he/she definitely went to Hell.
Prophet (SAW) said:

والله ما أدرى وأنا رسول الله ما يفعل بى

[roughly translated:] By Allah, I do not know what will be done with me, even though I'm the Messenger of Allah.
If Prophet (SAW) wasn't so sure, how can we claim to know about a person's fate? Do we have angels coming to us as reporters? :)
Side question: is it okay to organize a gathering to remember the deceased? Find the answer here.